Polyphenylene ethers, also known as polyphenylene oxides, are a class of polymers widely used in industry, especially as engineering plastics in applications requiring toughness and heat resistance. In recent years, there has developed an increasing interest in employing polyphenylene ethers in food packaging applications.
In many of these food packaging applications, it is essential that the polyphenylene ether be substantially free from materials which are volatile, have undesirable odors, or would otherwise harm the food. Various materials of this kind may be present in polyphenylene ether resins. They include dialkylamines, such as di-n-butylamine, which are components of the catalyst used in the preparation of polyphenylene ethers as described hereinafter. Also present may be by-products formed in the synthesis of the substituted phenols from which polyphenylene ethers are prepared. In the case of poly(2,6-dimethyl-1,4-phenylene ether) these frequently include 2,4,6-trimethylanisole (2,4,6-TMA), 7-methyldihydrobenzofuran (7-MDBF), 2,3-dihydrobenzofuran and 2,6-dimethylcyclohexanone. Conventionally, polyphenylene ether resins are manufactured by polymerizing the monomer in solution with a liquid aromatic hydrocarbon solvent. The resin may then be purified by a semicontinuous precipitation from the solvent with methanol.
However, the solvents used in the preparation of PPE and to precipitate it are ordinarily recycled, which results in a build up of odor components to a steady state level. Also, the methanol used to precipitate the resin from the solution is a relatively poor solvent for removing some of the odoriferous components.
Thus, the issue of unpleasant odors associated with polyphenylene ether resins has been a long standing problem with many plastics processors and has resulted in many attempts to reduce the odoriferous content in the resin.
Banevicius, in commonly assigned U.S. Pat. No. 4,906,700 reduces the content of phenolic odoriferous components during the initial production stage by distilling the liquid aromatic hydrocarbon reaction solvent prior to recycling to the polymerization zone or using fresh solvent. This has the effect of reducing the steady state buildup of impurities in the solvent and therefore also the polymer resin.
Other disclosures attempt to reduce the residual amine content by devolatilization during extrusion. Kasahara et al., U.S. Pat. No. 4,369,278; Newmark, U.S. Pat. No. 3,633,880; Banevicius et al., commonly assigned U.S. Pat. application, Ser. No. 07/291,534, filed herewith, now U.S. Pat. No. 4,992,222 and Bopp, commonly assigned U.S. Pat. application, Ser. No. 206,174, filed 6/13/88, now U.S. Pat. No. 5,017,656, all disclose various forms of vacuum vented extrusion as a devolatilization technique for removing volatile components from polymer resins. In Kasahara et al., the patentee teaches the optional use of water injection into the polymer melt to aid in devolatilization. Banevicius et al., describes an extrusion devolatilization process comprising at least two stages of water or steam injection into the polymer melt to effect a more complete removal of odoriferous components. However, the above processes require the addition of water into the polymer in melt form in a vented extruder. The present invention does not involve adding water to the polymer in melt form or an extrusion process, but rather adding water to the resin in particulate form and then boiling the water to distill a portion of the water and substantially all of the odoriferous components.
Also to be mentioned is Bunting et al. commonly assigned U.S. Pat. application, Ser. No. 07/291,563, filed herewith, now U.S. Pat. No. 4,994,217, which discloses a sequence of devolatilizing apparati, such as heat exchanger devolatilizers, to effect a reduction in odoriferous content. The patentees teach the injection of high pressure water or steam in a stripping zone but do not teach mixing of water and the resin and then distillation of the water.
Surprisingly, it has now been found that where water is admixed with a polyphenylene ether resin and a portion of the water is distilled, a low odor polyphenylene ether is produced. Distillation of the water from an aqueous resin slurry unexpectedly results in the production of polyphenylene ether resins exhibiting very low odor in human organoleptic tests.